Apparatus for use in aerodynamics investigations



Oct. 4, 1966 H, SHAPIRQ 3,276,252

APPARATUS FOR USE IN AERODYNAMICS INVESTIGATIONS Filed Jan. 5, 1964 DRIVE 4 PRES E E 5;@ GAU 3 I INVENTOR.

Ascher H. S opiro BY Attorn United States Patent Ofifice 3,276,252 Patented Oct. 4, 1966 3,276,252 APPARATUS FOR USE IN AERODYNAMICS INVESTIGATIONS Ascher H. Shapiro, Arlington, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Jan. 3, 1964, Ser. No. 335,700 6 Claims. (Cl. 73-147) The present invention relates generally to the field of rarefied gas dynamics and, more particularly to apparatus for and methods of investigating the interaction of low density, high speed, molecular streams with solid surfaces.

In the field of fluid dynamics it is oftentimes desirable to impact low density, high speed, molecular streams against solid surfaces. For example, satellites or space vehicles travelling at a high altitude encounter an environment where the mean free path of the atmospheric molecules is large compared with the dimensions of these devices. Consequently, the aerodynamic forces acting on these bodies and the heat transfer between them and the surrounding environment is determined by free-molecule or nearly free-molecule behavior, that is, circumstances in which intermolecular collisions near the body are comparatively few. In order to reproduce experimentally the operating environment of such a satellite, it is, therefore, necessary to create a low density, high speed molecular stream of a gas having a Maxwellian distribution of thermal velocities.

Besides the area of external flow, there is also interest in investigating internal free molecular flow as it occurs within ducts, and the like, in order to ascertain such factors as pressure distribution, temperature, velocity and mass flow rates.

Conventional wind tunnels do not lend themselves particularly well to the production of a free molecule type of flow primarily because of the requirement for the ratio between the size of the model and the size of the testsection nozzle. In order for a windtunnel nozzle to be capable of delivering a uniform and parallel gas stream capable of simulating flow relative to a body travelling through an otherwise stationary gas, the nozzle diameter should be of the order of ten to one hundred times the mean free path between molecular collisions. For the model to experience free-molecule flow of the gas stream, it should be a factor of the order of ten to one hundred times smaller than the mean free path between molecular collisions. Hence, the ratio between the size of the model and that of the nozzle should be at most of order of magnitude one to one hundred, and preferably one to one thousand or one to ten thousand. In the past, this relationship has been approached with available wind tunnels only by using very large tunnels and very small models. Often the model is only of wire size. This unfavorable ratio, it will be recognized, tends toward very costly facilities, severely complicates the study of the phenomena of interest and also renders the derivation of meaningful data from the model extremely difficult.

Also, the utilization of conventional wind tunnels to produce a high-speed gas flow at low pressure necessitates continuous evacuation of the apparatus to impart the necessary velocity to the gas molecules and also to keep the gas density low. Since the considerations stated previously favor large tunnels, the pumping apparatus required is correspondingly large and costly.

The present invention provides a new and novel arrangement for creating a high speed, low density, stream of gas with a Maxwellian velocity distribution for use in rarefied gas dynamic studies and experiments. Typical of such studies are the measurement of aerodynamic forces, the measurement of heat transfer rates and the investigation of interactions between molecules and surfaces.

According to the present invention, such a stream is generated by means of an unb'laded rotor spinning at a relatively high speed within a controlled gaseous atmosphere. More particularly, in one representative structural embodiment, this rotor has a toroidal cavity cut therein adjacent its peripheral surface and extending therearound. Communicating with this cavity and establishing an axis passageway thereto is a circumferential slot through which the supporting strut for the model passes. The model itself, which is rfixed to the stationary housing in which the rotor spins, is located within the cavity and this cavity, as will be seen hereinafter, performs somewhat in the manner of a rotating wind tunnel to create the flow conditions sought.

One of the significant advantages of the above arrangement is found in the fact that the model is stationary. The advantages of this configuration can best be appreciated from the consideration that the forces acting on the model and which have to be detected and precisely measured are generally proportional to the gas density and, accordingly, are very small. At this point, it might be mentioned that it has been known in the prior art to employ a rotating member with a model projecting from a point on its peripheral surface to simulate the effects thereon of high speed flight or flow. However, the disadvantages inherent in this mode of operation which centers primarily about the difficulty of extracting meaningful data information from the moving model are all the more magnified in a free molecule flow system where these signals are of feeble magnitude. In other words, since the electrical signal output of, perhaps, the end measuring transducers are of fairly small amplitude, these signals can be completely obscured by extraneous noise signals caused by dynamic rotor unbalance or by the commutating apparatus usually needed to extract the signals from the rotating apparatus. Moreover, with a stationary model, the measuring portion of the system can be initially adjusted to a fine degree of balance. Also, the instrumentation of the system is simplified and the precision of the measurements correspondingly enhanced.

In the present invention the rotor operates in a con fined gaseous atmosphere. Once this atmosphere is established by the evacuation of a suitable enclosure and the subsequent introduction therein of a proper amount of the selected gas, no further adjustments have to be made to this portion of the system throughout the duration of the experiment except for a small amount of pumping to account for minor leaks that might be present. This avoids the continuous pumping of vast quantities of gas required with wind tunnels and the :like.

Although the best application of the apparatus hereinafter to be disclosed perhaps resides in the field of wind tunnel investigations in the free molecule range, it can be used with advantage in the range of gas densities in which the fluid is a continuum. It is especially adaptable for investigations of supersonic flows in the range of normal aerodynamic densities. 'It can, in short, cover the entire range of densities from free molecule behavior to continuum behavior.

One of the important quantities it is necessary to simulate in a wind tunnel investigation in the free molecule range is the ratio of streaming speed to the mean molecular speed. In the conventional wind tunnel arrangement, alteration of this ratio, with a particular gas, requires a different nozzle structure or configuration for each speed ratio. In the present invention, this ratio can be adjusted either through changes of the rotor speed or through the choice of gas. For instance, if it is desired to attain the highest possible ratio of streaming speed to 9 molecular speed, one can use the maximum possible rotor speed together with a gas having a lower molecular speed. Such a gas would be one having a very large molecular weight inasmuch as the molecular speed varies as the inverse square root of the molecular weight.

It is accordingly a primary object of the present invention to provide apparatus for generating a low density, high speed, gas flow.

A still further object of the present invention is to provide apparatus for studying the impact of high speed, low density streams of gas on the surface of solid bodies.

A yet still further object of the present invention is to provide apparatus producing a stream of gas having a Maxwellian velocity distribution.

Another object of the present invention is to provide apparatus for use in wind tunnel investigations wherein the ratio of the streaming speed to the mean molecular speed of the gas stream can be varied.

Another object of the present invention is to provide apparatus for use in studying the aerodynamic forces, the heat transfer rates and the interaction between surfaces and molecules from the free molecule to the continuum density range.

A yet still further object of the present invention is to provide apparatus for generating a high speed gas flow wherein the molecular speed and density of the gas stream can be varied by simple procedures.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic drawing of the basic invention; and

FIG. 2 is an enlarged view of a portion of the rotor of FIG. 1.

Referring now to FIG. 1, which schematically illustrates in simplified form the operating principle of the present invention, it will be seen that the basic apparatus includes as its primary element a circular rotor 1 of high speed design adapted to be driven by motor 2 via driveshaft 3 supported by hearing 4. Rotor 1 and its associated drive are accommodated within an enclosure 5 which can be evacuated by means of a relatively small vacuum pump 6 connected in a discharge line 7 having a valve 8 included therein. Also coupled to this enclosure is a charging line 9 having a control valve 10 incorporated therein for introducing the desired gas into the apparatus and for maintaining a small constant flow to ensure that the composition does not change because of leakage.

Rotor 1 has a cavity 11 which has a shape that corresponds to part of a torus cut therein that eX- tends around its complete circumference. As perhaps best shown in FIG. 2, which is an enlarged view of a rim portion of this rotor, a slot 12 is also cut into the rotor to form a passageway between its peripheral surface 13 and the main cavity space. This slot permits gas molecules in the main portion of enclosure 5 to diffuse or otherwise pass into the toroidal cavity 11. Slot 12 also accommodates a strut 18 which, in the embodiment shown, is supported in a cantilever fashion from a base member 14 afiixed to an adjacent sidewall portion of enclosure 5. Attached to the other end of this strut is the model, here represented by a sphere 15, whose performance and characteristics in a high speed, free molecular stream are to be investigated.

In operation of the above apparatus, enclosure 5 is evacuated by pump 6. The gas selected for the investigation is next introduced via input line 7, and valve 8 in conjunction with valve 10 are adjusted until the desired gas pressure is established on pressure gauge 16. Then rotor 1 is set spinning at a high angular velocity by the energization of motor '2. It would be mentioned at this point that any acceptable drive arrangement can be employed with rotor 1. Also, as shown in FIG. 1, this drive can be positioned within enclosure 5. Alternatively, the rotor can be driven via an externally rotating, synchronous, magnetic field coacting with a suitable induction arrnature coupled to shaft 3. It should be appreciated that rotor 1, if made of the proper material, can itself serve as the rotating element of a synchronous motor. The rotor 1 may also be suspended magnetically by wellknown methods, thereby averting problems of bearings, of lubrication, and of contamination by oil vapor.

The linear speed attainable at the rotor tip is, of course, limited by the mechanical stresses in the material induced by centrifugal forces. It will be appreciated that the cross-sectional shape of rotor 1 should be so designed that it can reach the maximum speed consistent with the strength of the materials from which it is fabricated. With conventional materials and conventional forms of construction, linear speeds of about 2,000 feet per second are obtainable. With special materials, perhaps composite materials such as fiberglass, or the like, this speed can be extended up to 4,000 feet per second or more.

The gas molecules within enclosure 5 diffuse throughout this space and, as pointed out hereinbefore, some enter the toroidal cavity 11 by way of slot 12. Those molecules which strike the boundary surface 17 of this cavity are not reflected in a simple manner therefrom. Rather, they are rte-emitted as if they come from a location behind this surface from an imaginary gas at rest with respect to this surface. That is, their direction of re-emission is independent of their direction of incidence and all their incoming, tangential momentum is lost to the surface. The velocity distribution of these incident molecules will, in general, be other than Maxwellian. However, when re-emitted, they do have such a Maxwellian velocity distribution with respect to the surface from which they are emitted. Moreover, once rotor 1 is set spinning, the re-emitted molecules possess a mean mass velocity with respect to stationary model 15 equal to the peripheral speed of the rotor. Consequently, the stream of molecules emitted from the rotor play the same role as the high speed gas stream in a rarefied gas wind tunnel. Thus, model 15 finds itself in an environment where the gas molecules impinging thereon have a Maxwellian thermal velocity distribution superimposed upon a mean streaming velocity.

It will be appreciated that model 15 can be instrumented in accordance with the experimental data desired. For example, strut 18 can take the form of one arm of a knifeedge balance for measuring the forces exerted on model 15. Alternatively, a strain gauge or a quartz fiber suspension may be employed. Also, the orientation of rotor 1, which is shown as horizontal in FIG. 1, can be altered to suit the particular experiment.

Instead of measuring the aerodynamic forces acting on model 15, heat transfer measurements involving this model can be carried out provided the model has been instrumented with the proper temperature measuring and recording apparatus. Although the apparatus of FIG. 1 does not disclose instruments for measuring or otherwise recording the rotational speed of the rotor, it will be appreciated that any conventional technique can be used in this regard. Also, suitable control equipment can be utilized with the electrical drive in order to provide means for varying the rotational speed of rotor 1.

In order to allow a model, such as 15, whose dimensions are greater than the width of slot 12, to be placed within the cavity, this slot need only be widened at one particular point to the necessary size. Numerous other obvious ways of providing access to the cavity can be incorporated into the structure in order to accomplish this same result.

In FIG. 1 the cross section of the cavity space out in rotor 1 is circular. However, it should be recognized that other cross-sectional shapes can be used with the same results.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. Apparatus for use in studying the impact of gas molecules on solid surfaces comprising, in combination,

an enclosure having a particular gaseous atmosphere therein;

a rotor positioned within said enclosure;

means for driving said rotor;

said rotor having a peripheral slot extending completely around its rim portion and extending inwardly towards the center of said rotor;

said slot terminating in an enlarged portion whereby a stationary model positioned within said enlarged portion when said rotor is driven is impinged by gas molecules having a Maxwellian distribution of thermal velocities and a mean mass velocity equal to the peripheral speed of said rotor.

2. In an arrangement as defined in claim 1 wherein said enlarged portion having a shape which corresponds to part of a torus.

3. Apparatus for use in studying the impact of gas molecules and solid surfaces comprising, in combination,

an enclosure containing gas molecules at a controllable pressure;

a rotor positioned within said enclosure,

said rotor having a circumferential slot cut in its rim portion and a cavity formed within a body portion thereof, the shape of said cavity corresponding to part of a torus,

said cavity and said slot being in communication with each other whereby gas molecules may enter said slot and proceed into said cavity; and

means for rotating said rotor whereby those gas molecules which proceed into said acvity and strike the boundary surface of said cavity have imparted to them a Maxwellian distribution of thermal velocity and a mean mass velocity equal to the rim speed of said rotor.

4. Apparatus for use in aerodynamic and thermodynamic investigations comprising, in combination,

a rotor;

an enclosure surrounding said rotor and adapted to isolate said rotor from the surrounding environment; means forestablishing a particular gaseous environment for said rotor;

means for driving said rotor;

said rotor having a circumferential slot cut in its rim portion which extends inwardly a predetermined amount and then expands into an enlarged opening whereby a stationary member positioned therein when said rotor is driven is impacted with gas molecules from the surrounding environment which enter said enlarged opening through said slot, strike the boundary surface of said enlarged opening and are reemitted therefrom having a Maxwellian distribution of thermal velocities and a mean mass velocity equal to the rim speed of said rotor.

5. In an arrangement as defined in claim 4 wherein said enlarged opening has a substantially circular crosssectional shape.

6. In an arrangement as defined in claim 4,

a model positioned within said enlarged opening and adapted to remain stationary when said rotor is driven and set spinning.

References Cited by the Applicant UNITED STATES PATENTS DAVID SCHONBERG, Primary Examiner. 

1. APPARATUS FOR USE IN STUDYING THE IMPACT OF GAS MOLECULES ON SOLID SURFACES COMPRISING, IN COMBINATION, AN ENCLOSURE HAVING A PARTICULAR GASEOUS ATMOSPHERE THEREIN; A ROTOR POSITIONED WITHIN SAID ENCLOSURE; MEANS FOR DRIVING SAID ROTOR; SAID ROTOR HAVING A PERIPHERAL SLOT EXTENDING COMPLETELY AROUND ITS RIM PORTION AND EXTENDING INWARDLY TOWARDS THE CENTER OF SAID ROTOR; SAID SLOT TERMINATING IN AN ENLARGED PORTION WHEREBY A STATIONARY MODEL POSITIONED WITHIN SAID ENLARGED PORTION WHEN SAID ROTOR IS DRIVEN IS IMPINGED BY GAS MOLECULES HAVING A MAXWELLIAN DISTRIBUTION OF THERMAL VELOCITIES AND A MEANS MASS VELOCITY EQUAL TO THE PERIPHERAL SPEED OF SAID ROTOR. 